1. Field of the Invention
The present invention relates to a cordless power tool using a lithium battery, and more particularly, to a cordless power tool having an overcurrent protection circuit to prevent an overcurrent from flowing in a lithium battery.
2. Description of the Related Art
In a power tool such as an electric screwdriver, an electric drill, an impact power tool, rotational power generated from an electric motor is applied to a speed reduction mechanism to reduce rotational speed and then transmitted to a bit attached to the distal end of the tool. Conventionally, the power tools are provided with a cord for connection to a commercial AC power source and the motor is rotated by the power supplied from the commercial AC power source. Recently, however, cordless power tools have been widely used, in which an alkaline secondary battery such as a nickel-cadmium battery or a nickel-hydrogen battery is used as a power source.
A power tool operating with a large voltage requires a large number of battery cells. The nominal voltage of a nickel-cadmium battery cell is 1.2 V, a power tool operating with 14.4 V has to be equipped with a battery pack housing therein twelve battery cells connected in series. A power tool operating with 24 V has to be equipped with a battery pack housing therein twenty battery cells connected in series. Accordingly, there is a problem such that the tool becomes heavier as the operating voltage of the tool is larger.
On the other hand, organic electrolyte secondary batteries such as a lithium battery and a lithium-ion battery are large in their nominal voltage. Accordingly, the number of battery cells can be reduced to obtain the same operating voltage, resulting in reduction of the weight and size of the power tool.
The lithium batteries include a vanadium lithium battery and a manganese lithium battery, and use a lithium aluminum alloy in the negative electrode and an organic electrolyte. The lithium-ion battery generally uses lithium cobalt oxide in the positive electrode, graphite in the negative electrode, and an organic electrolyte. In the following description, organic electrolyte secondary batteries including a lithium battery and a lithium-ion battery will be referred to simply as a lithium battery.
The nominal voltage of a lithium battery cell is as high as 3.6 V. Thus, a voltage with three nickel-cadmium battery cells can be obtained with a single lithium battery cell. When the lithium battery is used as the power source of a cordless power tool, the use of the lithium battery can significantly reduce the number of battery cells. On the other hand, the charge/discharge cycle life of the lithium battery is significantly shortened if the battery is overcharged or overdischarged, or an overcurrent flows in the lithium battery.
U.S. Patent Application Publication No. 2003096158 discloses an overdischarge prevention control in which a field-effect transistor interposed between a battery and a motor is rendered OFF to stop discharging when the battery voltage falls below a predetermined value.
While it is possible to further perform an overcurrent prevention control in a similar manner to the overdischarge prevention control described above, the following inconveniences are caused if the overcurrent prevention control is performed by switching OFF a field-effect transistor whenever the overcurrent flows in the battery.
As shown in FIG. 1, when a DC voltage is applied to a DC motor M from a DC power source B through a switching element S, a current Ia given by the following equation flows in the motor M and the switching element S immediately after the switching element S is closed, that is, at the time of start-up.Ia=(V−E)/Rawherein V represents a voltage across the DC power source B; Ra, a resistance value of an armature winding of the DC motor M; and E, a counter electromotive force of the DC motor M.
At the time of start-up of the motor M, the counter electromotive force E is zero because the rotator of the motor M has not yet started rotating but is remained in a stationary state. For this reason, an overcurrent cannot be prevented from flowing in the motor M for a short time period. On the other hand, during use of a power tool such as an electric screwdriver or an electric power drill, the bit may bite into or seize a workpiece to be processed, with the result that the motor is temporarily brought into a locked state, that is, the motor stops its rotation.
FIG. 5 shows a current flowing in the armature winding of the motor M when the motor M comes into the locked state. In this case, since the counter electromotive force E of the motor M becomes zero, an overcurrent flows in the circuit including the switching element S and the motor M. If the overcurrent prevention control system is conFIGured to prevent an overcurrent from flowing in the circuit in all instances, the high level start-up current does not allow the motor M to start rotating. On the other hand, if the overcurrent prevention control system is conFIGured so that the high level start-up current is allowed to flow in the circuit, the DC power source cannot be protected from the overcurrent which may flow in the circuit during the use of the power tool. A lithium battery is unduly deteriorated by a high level current continuously flowing therein, causing the charge/discharge cycle life of the lithium battery to extremely shorten.